Composition and method for treating boiler water to prevent caustic embrittlement



' Patented May 11, 1943 COMPOSITION ING B TIC owl AND METHOD FOR'rnnmrwa'rna TO PREVENT C lnl G. Bird, Western Springs, and Evans,

111., and Carlyle Harmon and Jon-gen tion of Delaware, jointly NoDrawing. Applion March 4, 1941, Serial. No. 381,762

13 Claims.

This invention relates to the treatment of boiler water to preventcaustic embrittlement or intergranular attack of steel boiler plates.

The invention consists in the discovery that caustic embrittlement ofsteel boiler plates can be prevented by adding to the normal boilerwater a combination of chemicals consisting of a sufllcient minimumamount of sodium sulphlte, or its equivalent, and a sulphonated organicsubstance of suitable colloidal character and which has the property ofpreventing or retarding the oxidation of the sodium sulphite, such aslignin sulphonate.

The exact mechanism taking place in steel boiler plates undergoingcaustic embrittlement is not well established. It is known that failureoccurs only in plates under great mechanical stress when in contact withcaustic soda solution of the order of 20,000 to 40,000 parts permillion, together with a small amount of sodium silicate. In order toobtain such a high concentration of caustic it is nwessary that a smallleak exist so that the boiler water may escape very slowly. As the waterevaporates around such a leak it concentrates and deposits caustic soda,sodium sulphate and other dissolved materials present 11117118 water.

Previous attempts to prevent caustic embrittlement were based onmaintaining in the boiler water a certain ratio of sodium sulphate tototal alkalinity. The A. S. M. E. code recommends that for boilerpressures up to 150 lbs., one part of sodium sulphate be maintained foreach part of total alkalinity, expressed as sodium carbonate present inthe water. For pressures from 150 to 250 lbs. the recommended ratio isincreased to two parts 01 sodium sulphate for each part of caustic, andfor pressures greater than 250 lbs.

the recommended ratio is increased to three parts. It is known thatcaustic embrittlement takes place even when these recommended ratios aremaintained. It was this fact, and the difiiculties encountered inmaintaining such ratios that led us to seek a successful solution of theproblem of preventing caustic embrittlement.

In general, boiler waters are treated for at'least five differentpurposes: -(1) To prevent the formation of scale on the heated surfaces;(2) To prevent encrustation or deposition of the material in the feedlines, preheaters, etc.; (3) To prevent the corrosion of the metal; (4)To prevent foaming of the boiler water; (5) To prevent causticembrittlement or intergranular cracking of the heated surfaces. Itfrequently happens that a chemical or mechanical treatment may be usedfor one of these functions without having any efiect whatever on theother objects of the treatment.

The presence of dissolved oxygen in boiler water is responsible forpitting and corrosion and many methods have been proposed for removingthis dissolved oxygen. The use of sodium sul'phite for this purpose iswell known (see Oxygen removal from boiler water by Sodium Sulphite, byKobe and Gooding, Industrial 8: Engineering Chemist y. vol. 27,p.331-333, 1935) The following reaction takes place when the oxygen isremoved: 2NazSO3+Oz 2NazSO4. In this case the sodium sulphite is readilyconverted to sodium sulphate by the dissolved oxygen present in thewater, but caustic embrittlement is not prevented as the sodium sulphiteis converted in its reaction with oxygen;

The use of sulphonic acid salts or waste sulphite liquor products in thetreatment of boiler water is also well known (French Patent 619,040,French Patent 581,750) (Protecting steel against intercrystalline attackin aqueous solution, by W. C. Schroeder, A. A. Berk, and R. A. OBrien,A. S. M. E. Transactions for January 1938). However, the use ofligninsulphonate alone does not offer complete protection against causticembrittlement.

We have discovered that Sulphonated organic material such as sodiumlignin sulphonate has the property of inhibiting the oxidation of sodiumsulphite in aqueous solution. This is illustrated by the followingtable:

Table Per cent oxidation of sulphite after Composition of boiler water 5hours 16 hours 48 hours 1250 ppm 1 NaOH-50 ppm NBzSOa 20.0 52. 4 88. 31250 ppm N aOH-50 ppm Na SO 500 ppm Na-lignosulphonate '8. 9 l9. 5 33. 2625 ppm NaOH-25 ppm NazSOa" 23.0 55. l 86. 2 625 ppm NaOH 25 p m Nn2SOa250 ppm Na-lignosu phonate 10.6 20.0 30. 6 250 ppm Na0Hl0 ppm NmSOa" 20.6 5i. (i 85. i 250 ppm NaOH IO ppm NazS O3- 100 ppm Na-lignosulphonate9. 4 20. 8 35. 4

1 Abbreviation for parts by weight per million parts of water.

expected and efiective results not obtainable by the use of eithermaterial alone and that such mixture ofiers complete protection againstcaustic embrittleme'nt.

A thorough study of the reactions taking place between sodiumlignosulphonate and the other constituents in the boiler water such assodium carbonate, sodium hydroxide, sodium phosphate, sodium chloride,sodium sulphate, etc., indicates that lignin sulphonate releases smallquantities of sodium sulphite under such condition and thereby lessenssomewhat the dispersibility of the lignin sulphonate. However, fornormal boiler operations the amount of sodium sulphite thus released isnot sufiicient to fully prevent caustic embrittlement. We havediscovered that the addition of a predetermined quantity of sodiumsulphite and sodium lignin sulphonate t the boiler water effectivelyprevents caustic embrit tlement, as illustrated in the followingexamples:

Example 1.--Standard boiler test water containing 500 P. P. M. of sodiumhydroxide, 50 P. P. M. of sodium silicate, to which was added 200 P. P.M. of sodium lignin sulphonate, was

placed in a Schroeder test bomb (Association of American RailroadsCircular No. D. V. 989, entitled Intercrystalline Cracks in LocomotiveBoilers by W. C. Schroeder, A. A. Berk and R. A. OBrien), and thesolution maintained at a temperature corresponding to 250 lbs. gaugepressure.

The solution was in contact with a piece of highly stressed S. A. E.1020 boiler plate of inch thickness. After days one plate was crackedthrough 22% of, its thickness, the second plate through 25% of itsthickness and the third through of its thickness.

Example 2.--A similar test was made using a boiler water containing 500P. P. M. of sodium hydroxide, 50 P. P. M. of sodium silicate, and 10 P.P. M. of sodium sulphite. The plate was cracked through 100% at the endof the 30-day period. Similar tests with the sodium sulphite increasedfrom 10 up to 200 P. P. M. showed 63% cracking. Thus, apparently nopractical amount of sodium sulphite alone is satisfactory.

Example 3.Four similar tests were made using a boiler water containing500 P. P. M. of

sodium hydroxide, P. P. M. of sodium silicate,

and 200 P. P. M. of sodium lignin sulphonate, to which 10 P. P. M. ofsodium sulphite was added. After '30 days in the test block the steelsample showed no sign of cracking whatever. Similar tests with increasedamounts of sodium sulphite also showed complete protection. Since thewater used in these tests contained approximately 8 P. P. M. ofdissolved oxygen there was enough oxygen present to react with 63 P. P.M. of sodium sulphite to convert *it to sodium sulphate. Such oxidationwas prevented by the presence of the sulphonate organic matter, such assodium lignin sulphonate, which inhibited oxidation of sodium sulphite.

Example 4.A similar test was made usinga boiler water containing 500 P.P. M. sodium hydroxide, 50 P. P. M. sodium silicate, 500 P. P. M. sodiumchloride, and 1500 P. P. M. sodium sulphate. The 1020 steel sample wascracked through 61% of its thickness in 30 days.

Example 5.-A similar test was made with the same solution as Example 4with the addition of 200 P. P. M. of lignin sulphonate. The platecracked through 58% of its thickness in 30 days. Example 6.A similartest was made using the same solution as in Example 4 but with theaddition of 90 P. P. M. of lignin sulphonate and 10 P. P. M. of sodiumsulphite. The plate showed no cracking whatever in 30 days.

We have found that the addition of about 10 to 50 parts by weight of asulphite such as sodium sulphite, or its equivalent, and 50 to 300 partsby weight of a colloidal sulphonated organic compound selected from. thegroup consisting of lignin sulphonate, and sulphite cellulose liquors,such as sodium lignin sulphonate, per million parts of boiler water isan effective concentration for preventing caustic embrittlement undernormal boiler operations. We prefer to regulate the amount present inthe boiler water by adding known amounts of the constituents to the feedwater or directly to the boiler, taking into consideration the number oftimes the feed water is concentrated in the boiler, this beingdetermined by means well known to those proficient in the art of boilerwater treatment. For example, by means of the chloride ratios, per centblowndown, etc.

Since the exact nature of the reaction causing embrittlement is notknown it cannot be said with certainty how a small quantity of sulphiteand lignin sulphonate can stop such reaction. In addition to the ligninsulphonate preventing the oxidation of the sulphite, the sulphite may inturn act to maintain the colloidal dispersion of the lignin sulphonateso that it can form a protective coating for the steel, and thus protectit from the action of the concentrated alkali. Another possibility isthat the sulphite, or the lignin sulphonatemay affect the nature of theoxide film present on the stressed steel plate and thus prevent theconcentrated caustic from attacking the intergranular structure.

While the specific example have referred to sodium sulphite we do notwish to be limited to sodium alone. The active ingredient is thesulphite ion which may be supplied by adding any suitable compound,such, for example, as alkali or alkaline earth sulphites, or bisulphiteor other materials that after addition to hotalkaline boiler water willyield sulphite ions.

We prefer to employ as the sulphonated organic material, sodium ligninsulphonate but other sulphonated compounds such as magnesium or calciumligninsulphonate or concentrated pa-' per mill waste sulphite liquorsmay be used.

From the chemical nature of-quebracho, cutch and from the claims made byother investigators regarding the protection against causticembrittlement offered by these materials it might be reasoned that theywould react in a similar manner with bisulphite. However, ourinvestigation indicates that quebracho actually accel erated theoxidation of bisulphite in aqueous so,- lution and that cutch has littleor no action on the oxidation.

Our invention can be conveniently practiced by first preparing suitableboiler water treating compositions containing the usual causticalkalinity producing compounds. such as caustic and changes of ourspecific embodiments can be steam boilers which comprises adding to thenormal feed water containing caustic alkalinity producing compounds 9.mixture of a sulphite selected from the group consisting of alkali andalkaline earth metal sulphites and a colloidal sulphonated organiccompound selected from the group consisting of alkali and alkaline earthlignin sulphonates and sulphite cellulose liquors insumcient-proportions to inhibit caustic embrittlement and thereafterintroducing the water in a boiler.

2. The method of preventing caustic embrittlement in a steam boilerwhich comprises adding to the normal feed water containing causticalkalinity producing compounds a mixture of a sulphite selected from thegroup consisting of alkali and alkaline earth metal sulphites and alignin sulphonate selected from the group consisting of alkali andalkaline earth metal lignin sulphonates in suflicient proportions toinhibit 7. A composition for treating boiler water to prevent causticembrittlement comprising an inorganic sulphite selected from the groupconsisting of alkali and alkaline earth metal sulphites, aiignosulphonate selected from the group consisting of alkali andalkaline earth metal lignin sulphonates and a caustic alkalinityproducing compound for normal boiler operation in suitable proportionswhen dissolved in boiler water consisting of alkali and alkaline earthmetal lig-' operation of said steam boiler and thereafter introducingthe water in a boiler. I

3. The method of preventing caustic embrittlernent in steam boilerswhich comprises adding to the normal feed water containing causticalkalinity producing compounds a mixture of sodium sulphite and sodiumlignin sulphonate in sufilcient proportions to inhibit causticembrittlement and thereafter introducing the water in a boiler.

4. The method of treating boiler water used in steam boilers whichcomprises adding to the normal feed water a mixture of a sulphiteselected iromthe group consisting of a and alkaline earth metalsulphites and a lignin sulphonate selected from the group consisting ofalkali and alkaline earth metal lignin sulphonates in sumcientproportions to prevent caustic embrittlement.

5. A method of treating boiler water used in steam boilers whichcomprises adding to the normal feed water a mixture of a sulphiteselected from the group consisting of alkali and alkaline earth metalsulphites and a lignin sulphonate selected from the group consisting ofalkali and alkaline earth metal lignin sulphonates in sufficientproportions to result in -a concentration water to preventcausticembrittlement duringbciler operations.

to yield in solution about 10 to 50 parts by weight of sulphite andabout 50 to 300 parts by weight of lignosulphonat per million parts ofwater.

8. A composition for treating boiler water to prevent causticembrittlement comprising an inorganic sulphite selected from the groupconsisting of alkali and alkaline earth metal sulphites and alignosulphonate selected from the group nin sulphonates in suitableproportions to yield in solution 10 to 50 parts by weight of sulphiteand 50 to 300 parts by weight of lignosulphonate per million parts ofwater. p

9. A composition for treating boiler water to prevent causticembrittlement comprising sodium sulphite and a sodium lignosulphonate insuitable proportions to yield in solution 10 to 50 parts by weight ofsulphite and 50 to 300 parts by weight of lignosulphonate per millionparts of water.

10. A composition for treating boiler water to prevent causticembrittlement comprising an inorganic sulphite selected from the groupconsisting of alkali and alkaline earth metal sulphites and a colloidalsuphonated organic compound selected from the group consisting of alkaliand alkaline earth lignin sulphonates and sulphite cellulose liquors insuitable proportions to yield in solution 10 to 50 parts by weight ofsulphite and 50 to 300 parts by weight of a colloidal sulphonatedorganic compound per million parts of water.

11. A process ,of inhibiting oxidation of an aqueous solution of sodiumsulphite which comprises the step of adding to the solution smallamounts of lignin sulphonate selected from the group consisting ofalkali and alkaline earth metal lignin sulphonates.

12. A composition comprising sodium sulphite inhibited against oxidationby the presence of lignin sulphonate selected fromthe group consistingof alkali and alkaline earth metal lignin sulphonates.

. 1-3. A composition comprising a sodium sulphite inhibited againstoxidation by the presence of a colloidal sulphonated organic compoundselected from the group consisting of alkali and alkaline earth ligninsulphonates and sulphite cellulose liquors.

'PAUL G. BIRD.

WILSON EVANS.

CARLYLE ON.

JGRGEN RICHTER SALVESEN.

